The choice of Stainless steel material for brewery

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Stainless steel is widely used in the food and beverage industry due to its high temperature resistance, corrosion resistance and hygienic properties. Compared to other areas such as oil and gas production, beer brewing vessels and pipes are regularly cleaned using CIP (site cleaning). In order to obtain the best cleaning results, good surface treatment of containers and pipes is critical. Since the 1960s, industrial beer brewing processes used to manufacture containers and tanks have often used stainless steel AISI 304, or AISI 316, and duplex stainless steel 2205. The corrosion resistance of 2205 stainless steel is comparable to that of AISI 304 while strength is higher, and it is not easy to produce chloride cracking when the temperature is higher than 60℃. Mashed malt, wort, and beer do not corrode stainless steel, even at boiling point. However, the cold-worked stainless steel is prone to chloride cracking when used above 60℃. In general, the brewing solution also does not corrode AISI 304 stainless steel. Only in beer brewing using soft water, AISI 316 stainless steel can be chosen due to the high chloride content.

Chloride cracking may occur in thin-walled tubes and vessels due to their susceptibility to tensile stress. If the vessel leaks, it is often due to substandard welding quality or high fatigue load. CIP (field cleaning) does not corrode stainless steel, but under extreme conditions may cause chloride cracking on stainless steel with a high degree of cold forming. Fatigue corrosion and stress corrosion cracking failure mechanisms are similar. An example of fatigue corrosion in a saccharifying tank is the opening of a grain bin. After mashing and heating, the grains are separated from the wort and discharged through the opening of the grain barn. The impact and high load from the discharged grain produce fatigue corrosion cracks along the weld edge in the area directly opposite the mouth of the warehouse. The leakage in some places is due to poor quality. The wort container may crack from the outside to the inside due to chloride cracking and heat fatigue. If there is a high welding internal stress during steam-heated spiral pipe welding, cracking may occur throughout the stainless steel vessel wall.

Sensitivity of stainless steel

AISI 304 or 316 stainless steel has a carbon content of < 0.08% and can be sensitized if exposed to 500 ~ 800 ℃ for a given period of time, which may occur during welding. Therefore, welding causes sensitization of the “heat-affected zone” along the weld.

Sensitization will lead to the formation of chromium carbide at grain boundaries, resulting in poor chromium at grain boundaries, easy to cause intergranular corrosion of stainless steel in the case of thick tube wall (BBB 0 2 ~ 3mm). In order to avoid this situation, often choose “weldable steel”: such as L-grade steel, such as 304L, 316L, which carbon content is less than 0.03%; Titanium stabilized steel: 321,316 Ti.

 

Surface treatment

For the corrosion resistance of stainless steel, the weld quality and heat affected zone, surface roughness and the condition of the protective oxide layer are important. The surface condition of stainless steel is particularly important for the food and beverage industry and pharmaceutical industry. Corrosion problems in breweries are often caused by uneven surface conditions. During fabrication (welding, heat treatment, grinding, etc.), the passivated chromium oxide layer is damaged, thus reducing the corrosion resistance. Insufficient protective gas used in stainless steel welding will lead to the formation of a hot tempering color. These porous thermal tempering colors are composed of various oxides that tend to absorb ions such as chloride ions, reducing corrosion resistance and failing to protect the base metal.

If thermals or other types of contaminants are unacceptable, some kind of metal finish must be used to address them. Pickling or passivation can remove the old oxide layer, heat back color and other contaminants, thus allowing the passivated chromium oxide film to completely recover. The most common pickling process is to immerse stainless steel tubes in a mixed acid solution of nitric acid and hydrofluoric acid, which can also be accomplished by a spray or piping rinse system. Although the surface of the stainless steel is active after pickling, a passivation film can be formed within 24 hours due to the reaction of chromium with oxygen in the air, but in some cases, passivation is chemically facilitated by the use of nitric acid.

 

Welding

Welds and heat-affected zones are often the cause of corrosion. For breweries and other food industries, defects in welds, such as lack of penetration, are of Paramount importance, causing hygiene and sterilization problems. Engineers and buyers often identify inappropriate welding conditions and welding procedures that cannot be performed correctly. The result is poor quality welds and surface conditions in the construction that must be completed.

Thermal reheating is caused by light being absorbed into a transparent oxide layer, due to the different thicknesses of the oxide layer. Because the colors have different refraction coefficients, the blue-looking oxide layer can only reflect blue light and absorb other light. Thicker oxide layers have more holes than fully transparent thin oxide layers, therefore, thicker oxide layers will reduce the corrosion resistance and non-adhesion of stainless steel. For most standards, a light straw color of heat back is acceptable; All other heat-back colors such as red and blue are unacceptable. The pharmaceutical industry does not allow hot tempering.

The geometry of the weld shall be as regular as possible. Qualified welds will not damage the metal surface of the substrate. Corrosion often begins inside a tiny pinhole at the beginning/end of a weld.

Theoretically, there are no tiny pinholes, looseness, or other bumps at the start/end. Good weld penetration is very important. Piping must be well symmetrical and the width of the weld shall be fixed.

 

Surface roughness

Surface roughness affects the hygiene and corrosion properties of stainless steel. The corrosion resistance of the electropolished surface is the best, followed by the mechanically polished surface. In general, the beer industry and the food industry do not force the use of electropolished surfaces, however such surfaces, thus achieving excellent sanitary conditions and easy cleaning. Most pipes are bright annealed during manufacturing. Because the bright annealing process greatly improves the quality, pickling inside such pipes is often not performed unless the material surface has a severe heat back color or is contaminated with iron. Stainless steel sheet often has 2B surface, they have good surface performance. In breweries, thin-walled, straight-welded stainless steel pipes are most commonly used, with 2B finishes and sometimes another finish (brush or polish) on the exterior. Stainless steel extruded tubes are not commonly used in breweries; they are used for high-pressure purposes.

Comparison of 301, 301L, 301LN steel plate

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301 stainless steel is a type of austenitic stainless steel with a high work hardening rate. Its tensile strength can be up to 1300MPa or more. 1/16 hard to full hardening cold-rolled 301 plates are available and maintain sufficient ductility under 1/2 hardening conditions. It can be used for aircraft components, structural components of buildings, especially railway carriage components after rolling or bending. The 3/4 hardening to full hardening cold-rolled sheets should be used for simple component designs that require high wear resistance and elasticity. The 301L and 301LN are low carbon versions and high nitrogen versions of the 301. If better ductility is required or thick section profiles are to be welded, the low carbon 301L is preferred. The higher nitrogen content of 301Ln can compensate for the lower carbon content. They are specified in ASTM A666, JIS G4305, and EN 10088-2.

 

Chemical composition of 301, 301L, 301LN

Grade C Mn Si P S Cr Ni N
301 ≤0.15 2.0 1.0 0.045 0.03 16.0-18.0 6.0-8.0 0.1
301L ≤0.03 2.0 1.0 0.045 0.03 16.0-18.0 6.0-8.0 0.2
201LN ≤0.03 2.0 1.0 0.045 0.03 16.5-18.5 6.0-8.0 0.07-0.2

 

Mechanical property of 301, 301L, 301LN

301 Tempering

ASTM A666

 Tensile strength, Mpa Yield strength 0.2%, Mpa Elongation (in 50mm)thick>0.76mm Hardness, Rockwell
Annealed 515 205 40 /
1/16 hard 620 310 40 /
1/8 hard 690 380 40 /
1/4 hard 860 515 25 25-32
1/2 hard 1035 760 18 32-37
3/4 hard 1205 930 12 37-41
Full hard 1275 965 9 41+

 

Specification of 301, 301L, 301LN

Grade UNS No Euronorm JIS
No Name
301 S30100 1.4319 X5CrNi17-7 SUS 301
301L S30103 / / SUS 301L
201LN S30153 1.4318 X2CrNiN18-7 /

Corrosion resistance

Similar to 304 stainless steel, it has good corrosion resistance in normal temperature and mild corrosion applications.

Heat resistance

Good oxidation resistance to temperatures up to 840°C (intermittent use) and 900°C (continuous use). Exposure above 400°C causes gradual loss of work hardening effect, and the strength at 800°C is equivalent to 301 of annealing. Under creep conditions, the strength of work-hardened 301 even decreases to lower than that of annealed 301.

Solution (annealing) treatment

Heated to 1010-1120°C and rapidly cooled and annealed at approximately 1020°C. Heat treatment will not harden it.

Cold working

301 stainless steel and its low carbon version 301L for the need of high strength occasions. It has a very high work hardening rate of about 14MPa/%Ra (for each 1% of cold working surface reduction, tensile strength increases by 14MPa), cold rolling and cold forming can achieve very high strength, a portion of the strain hardening austenite converted to martensite. 301 is not magnetic under annealing conditions, but strong magnetic after cold working.

Welding

301 can be used for all standard welding methods and mostly 308L filler metal can be used for 301 weldings. 301 stainless steel welds must be annealed for optimal corrosion resistance, whereas 301L or 301Ln welds do not require annealing. Welding and post-weld annealing both reduce the high strength caused by cold rolling, so spot welding is often used to assemble cold-rolled 301 parts which has a small heat-affected zone and the strength of the whole part is almost not reduced.

Typical applications

Rail vehicle structural parts-roll forming, bending forming, or stretch forming into profiles, also in sheet. Aircraft fuselage, road trailer, car hub cap, wiper holder, toaster spring, stove fixture, screen frame, curtain wall, etc.

 

 

Dual grade stainless steel 304 /304L, 316/316L

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Austenitic stainless steels are the most widely used stainless steels, accounting for about 75% of the total stainless steel consumption. The rapid development of the chemical industry and petrochemical industry has put forward higher requirements for the corrosion resistance and strength of stainless steel. For example, the 304/304L dual grades stainless steel means it has lower carbon content, that’s less than 0.03%, meeting the 304L grades, while its yield and tensile strength are higher than the lower limit of 304 stainless steel, the stainless steel can be defined as 304/304L dual grades stainless steel, that is, its chemical composition meets that of 304L, and mechanical properties to meet the requirements of 304 stainless steel. Similarly, a stainless steel sheet can be 304/304H dual certified because it has enough carbon content to meet the 304H (minimum 0.040%) requirement and also meets the 304H grain size and strength requirements, there are 316/316L and other dual grades of stainless steel.

The most important is the difference in carbon and the resulting strength. Carbon is an effective austenitic stabilizing element and can be considered as an impurity or an alloying element that improves the strength of stainless steel, especially at high temperatures. The carbon content in most austenitic stainless steels is below 0.02% ~ 0.04%. In order to have good corrosion resistance after welding, the carbon content of low carbon grade stainless steel is controlled below 0.030%. In order to improve the high-temperature strength, the high carbon or “H” grade carbon content is maintained at 0.04% or slightly higher.

The smaller carbon atoms in the face-centered cubic structure are in the lattice gaps between the larger Cr, Ni and Mo atoms, which limit the dislocation motion, hinder the ductility deformation and strengthen the stainless steel. Under the condition of rising temperature such as in the welding process, carbon has a strong tendency to precipitate chromium in stainless steel matrix with chrome-rich carbide, and the second phase tends to precipitate at grain boundary rather than grain center, so chromium carbide is easy to form at the grain boundary.

Chromium is a necessary element for enhancing the corrosion resistance of stainless steel, but chromium carbide is removed from the stainless steel matrix, so the corrosion resistance here is worse than the rest of the stainless steel matrix. Increasing the carbon content can extend the temperature range, so that the time of sensitization or corrosion resistance loss is shortened, reducing the carbon content can delay or completely avoid the formation of carbide in welding. Low carbon grades such as 304L and 316L carbon content less than 0.030%, most of the higher alloyed Austenite grades such as 6%Mo stainless steel carbon content is less than 0.020%. To compensate for the decrease in strength due to the decrease in carbon content, another interstitial element nitrogen is sometimes added to strengthen the stainless steel.

Dual-grade stainless steel has both the high strength of conventional stainless steel and the corrosion resistance of ultra-low carbon stainless steel. It can solve the problem of weak welding joint performance of most Austenitic stainless steel, has been widely used in low-temperature LNG receiving station equipment and large diameter pipeline. The price of dual-grade stainless steel is basically the same as ultra-low carbon stainless steel. Now several Chinese steel mills can supply the grades for mature market, any interested, please contact us.

 

What’s Super 304H steel?

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With the development of ultra-supercritical units, the high-temperature strength of traditional 18-8 Austenitic stainless steels (such as TP304H steel) has been unable to meet their needs with steam parameters of 600℃. For this reason, Japan Sumitomo Metal Corporation has developed new materials for the boiler heating surface pipeline of the unit, like TP347HFG steel, SUPER304H steel and HR3C steel. Super 304H steel is a new type of 18-8 steel, mainly used in the manufacture of superheater and reheater of ultra-supercritical boilers whose metal wall temperature does not exceed 700 ℃. At present, Shasqida Mannesmann (formerly DMV Company) in Germany also produces similar steel tubes, with a grade DMV 304HCU.

Super304H steel is the steel by reducing the content of Mn, Si, Cr and Ni-based on TP304H steel, which adding 2.5% ~ 3.5% Cu and 0.30%~0.60% of Nb and 0.05%~0.12% of N, so that to produce the diffusion precipitation phase and copper-rich strengthened phase in service, occurs precipitation strengthening with NbC(N), NbCrN and M23C6, which greatly increases the allowable stress at service temperature, and the allowable stress at 600 ~ 650℃ is 30% higher than that of TP347H steel. The steam oxidation resistance of the steel is comparable to that of TP347HFG steel and significantly better than that of TP321H steel. It has been listed in ASME Code Case 2328-1, ASTM A-213 Standard, the number is S30432.

 

The Chemical Composition of Super 304H

C Si Mn P S Cr Ni N Al B Nb Cu V Mo
0.08 0.21 0.79 0.03 0.001 18.42 8.66 0.11 0.007 0.004 0.5 2.77 0.04 0.35

 

The Mechanical Property of Super 304H

Yield strength, Mpa Tensile strength, Mpa Elongation, %
360/350 640/645 58/60

 

Due to the high steam parameters of ultra-supercritical units, the oxidation resistance of steel used in high-temperature pressure parts of power plants becomes very important. Generally, the inner wall of the super 304H steel pipe is shot blasting to improve the anti-steam oxidation performance. A 30μm thickness shot blast layer was formed on the inner surface of the steel tube and its microstructure was refined compared with that of the non-shot peening steel tube. After the steam oxidation test at 650℃ and 600h, the oxide layer thickness of the steel tube treated by the shot blast is thinner and denser, and the steam oxidation resistance of the steel tube is improved. Currently, several leading steel mills in China have produced a similar grade 10CrL8Ni9NbCu3Bn, specified in GB 5310-2008, which is currently used in several ultra-supercritical unit projects in China.

Is 304 stainless steel magnetic?

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Ordinary consumers have some misunderstandings about stainless steel, they think that the magnetic stainless steel is not qualified 304 stainless steel. As we know, according to the structure under room temperature，stainless steel can be divided into Austenite such as 201, 304, 321, 316, 310, Martensite or Ferric such as 430, 420, 410. Austenites are non-magnetic or weakly magnetic and Martensite or ferrite are magnetic. 304 is a representative grade of the austenitic stainless steel, it has excellent workability, weldability and corrosion resistance, account for 60% of the world consumption of stainless steel, generally, it is no magnetic, but sometimes it is magnetic or weak magnetism caused by smelting chemical composition fluctuations or processing, but we cannot think this is fake or substandard, what reason is this?

304 is metastable stainless steel, is a single austenite structure after annealing state, with no magnetic. Smelting composition segregation or improper heat treatment will produce a small amount of martensite or ferrite structure, so with a weak magnetic. In addition, after the cold processing deformation (such as stamping, stretching, rolling, etc.), part of the austenite structure also underwent phase change (general mutagenesis into martensite) and with magnetic.

For example, in the same batch of steel strips, the outer diameter of 76mm steel pipe has no obvious magnetic while the outer diameter of 9.5mm steel pipe has obvious magnetic. The magnetic properties of the square rectangular tube are more obvious because the cold bending deformation is greater than that of the round tube, especially in the bending part.

Most of the water sink is made of 304 stainless steel. Many consumers judge it is made of 304-grade stainless steel according to whether the water tank is magnetic or not. At present, there are many kinds of processing technology for the sink, such as welding forming, integral tensile forming, etc., if used 304 material welding forming, is generally annealed after the plate processing, will not be magnetic or weakly magnetic (because of the surface treatment of the sink); One of the water tank drawing molding needs to go through several stretching, general annealing and then stretching (annealing increases the cost, and 304 is not necessary to anneal again), it will be magnetic, that is a very normal phenomenon.

304 stainless steel VS 403 stainless steel

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Grades 304 and 430 are commonly used stainless steel materials. 304 stainless steel is a general type of chromium-nickel austenitic stainless steel, the density of 7.93 g/cm3, also known as 18/8 stainless steel, is 300 series of stainless steel is the most commonly used steel. It can withstand high temperature 800℃, has good processing performance and toughness, widely used in the requirements of good comprehensive performance (corrosion resistance and molding) equipment and parts. 304L is a low-carbon version of 304, which does not require post-weld annealing, so it is widely used for thick gauge parts (approx. 5mm and above). The higher carbon content of 304H can be used at high temperatures. The annealed austenite structure also gives these grades excellent toughness, even at low freezing temperatures.

Low carbon high chromium 430 is one of the most common ferritic stainless steels, has good corrosion resistance, also known as 18/0 or 18-0, is one of the 400 series of stainless steels. It can be made slightly strengthened by cold working, but the low-temperature toughness is poor, and generally can not be hardened by heat treatment. Its thermal conductivity is better than austenite, the coefficient of thermal expansion is smaller than austenite, heat resistance fatigue, the addition of stabilizing element titanium makes the welding seam part of the mechanical property is good, can be used for building decoration, fuel burner parts, household appliances, household appliances parts. 430F is a kind of steel with free cutting performance on 430 steel, mainly used for automatic lathes, bolts and nuts, etc. 430LX adds Ti or Nb in 430 steel, reduces the content of C, and improves the processing performance and welding performance. It is mainly used for hot water tanks, heating water systems, sanitary appliances, household durable appliances, bicycle flywheels, etc.

 

According to ASTM A240- Specifications for chromium and chromium-nickel stainless steel plates, sheets and strips for pressure vessels and general purposes, 430 stainless steel shall contain less than 0.12% carbon, between 16-18% chromium, and less than 0.75% nickel, the difference between 304 and 430 as shown in the table below:

Chemical composition comparison 

UNS C Mn P S Si Cr Ni Mo
S30400 0.07 2.00 0.045 0.03 0.75 17.5-19.5 8.0-10.5 /
S43000 0.12 1,00 0.04 0.03 1.00 16.0-18.0 0.75 /

 

Mechanical property comparison

Grades Yield strength, Mpa Tensile strength, Mpa Elongation in 2 /50mm, min, % Hardness, HBW
304 205 515 40 183
403 205 450 22 201

 

To sum up, they differ mainly in the following items:

  • Corrosion resistance: The corrosion resistance of 304 stainless steel is better than 430. Because 430 stainless steel contains 16.00-18.00% chromium, basically does not contain nickel, 304 stainless steel contains more chromium and nickel;
  • Stability: 430 stainless steel is ferrite form, 304 stainless steel is austenite, more stable than 430 stainless steel;
  • Toughness: The toughness of 304 is higher than 430 stainless steel;
  • Thermal conductivity: The thermal conductivity of ferrite 430 stainless steel is like 304 stainless steel;
  • Mechanical properties: 430 stainless steel welding seam mechanical properties than 304 stainless steel is better because of the addition of stable chemical element titanium.